In railway transportation systems it is often desirable to transmit information to a rail vehicle by the use of cab signaling. The information desired to be transmitted is encoded into a cab signal current which is transmitted to the vehicle through the rails. When the cab signaling current reaches the vehicle, the signal information may be detected and the information utilized by the vehicle. Some of the information transmitted may be of a nature that is desirable to be known by those on board the vehicle, and may be information that is redundant with wayside signaling information. However, in some instances it may be desirable that the cab signaling information transmits data to the vehicle which is vital to the operation of the vehicle, such as speed commands, and track conditions which effect the operation of the vehicle. This information can be received by the vehicle through an antenna usually positioned in front of the lead axle which inductively couples to the cab signal current which is in the rail in front of the lead axle. The lead axle tends to act as a shunt between the rails and therefore the positioning of the cab signal antenna or inductive coupling is usually done in close proximity to, but in front of the lead axle. This inductively coupled pick-up is an adequate means to receive cab signal information by vehicles which are not powered by frequency varying electric motors. Frequency varying electric drive motors, such as AC propulsion motors used on board locomotives, utilize high current variable frequency electric power. Frequency varying electric drive motors, such as AC locomotive motors, can produce a high level of electromagnetic interference to the cab signal. Cab signal frequencies in the rail current are usually at frequencies of 60 hertz and 100 hertz. The AC drive locomotives use variable frequency, variable amplitude control techniques to drive three-phase traction motors. These propulsion motors draw currents in the order of magnitude of hundreds of amperes. In addition, over the speed range of operation of the locomotive, the frequency range of the propulsion motor current varies over a broad range. At certain speeds and/or propulsion currents the locomotive motor current will have frequency components that will be close to or equal to the cab signal frequency, such as 60 hertz and 100 hertz. Because the locomotive routinely operates over various speed ranges the interference presented by the AC propulsion current can be expected to be encountered at any time during operation, and often enough so as to present a problem to the reliable receipt of AC track signal information.
One solution to avoid the interference between the cab signal system and the AC propulsion system would be to operate the two systems at different frequencies. If the frequency band of the AC propulsion were to be outside the cab signal frequency bands the problem created by the concurrent operation of both systems would be eliminated. However, the present cab signaling frequencies have been utilized for many years and much of the existing equipment operates at those frequency ranges. It would be impractical to change all of the existing cab signal equipment to different frequencies. Similarly, in the AC locomotive propulsion equipment presently utilized, the horsepower and speed ranges demanded by AC traction motors makes the utilization of frequencies between 50 and 100 highly desirable. Therefore it is desirable to have a system which would permit compatibility between existing cab signaling equipment and present AC propulsion motor vehicle drive.